Many of the devices currently commercially available for introducing catheters into the lumen of a blood vessel or other physiological structure are utilized in practicing the Seldinger technique. According to this technique, a wall of the vessel to receive the catheter is pierced by a hypodermic needle, and a flexible guidewire is threaded through the needle into the vessel and is positioned as desired. The needle is then removed and a catheter is inserted over the guide wire into the vein or artery, following the guide wire to the desired position.
A modification of this technique employs a cannula carried by the needle and which enters the vessel with the needle. The guidewire is passed through the needle, leaving the cannula in place about the needle. However, use of such cannulas can result in trauma to both the vessel and the tissue through which the cannula must pass to reach the vessel. The trauma can be reduced by providing a taper on the cannula such that the opening in the vessel is gradually expanded as the cannula is advanced, but such devices (commonly referred to as dilators) as are currently available still cause significant damage as the needle and cannula are passed through the tissue into the vessel. This trauma is primarily due to an abrupt transition in diameter between the dilator and the hollow needle used for cutting a passage through the blood vessel wall. As a hollow needle punctures a vessel, the vessel tends to collapse locally. As the forward shoulder of the dilator contacts the outer wall of the vessel, it cannot simply pass through the hole created by the needle because its outer diameter is greater than that of the needle. Thus, it must be forced through the passageway, damaging tissue as it does so, and pinching the blood vessel walls together. The sharp needle end that extends beyond the end of the dilator thus pierces both vessel walls (rather than only the outer wall) as the dilator forces its way into the vascular channel. In common practice, both walls of the vessel are punctured, the needle then being retracted from the inner wall back into the lumen of the vessel. This causes unnecessary damage to both the blood vessel and surrounding tissue and may cause a hematoma to form below the second puncture; also, the plugs of vascular tissue formed by "coring" as the needle end passes through both vessel walls may tend to plug the lumen of the needle.
Accordingly, it would be desirable to have a needle and dilator assembly which could be used to place the dilator in the lumen of the vessel without necessitating puncture of both the outer and inner vessel walls to guarantee proper placement. Further, it would be desirable to have a device which would allow smooth and relatively atraumatic entry of the dilator into the vascular channel and in which little damage is cased by the tissue in its path.